1. Field of the Invention
The present invention relates to a high power amplifier, in particular to a predistortion linearizer for a high power amplifier that is capable of improving a nonlinear character of a high power amplifier.
2. Background of the Related Art
In general, a high power amplifier is used to increase a power of an input RF signal, and an ideal high power amplifier increases size of the input RF signal without distorting it.
Because the high power amplifier comprises active elements having nonlinear characteristics, however, distortion components are inevitably included in the output of the high power amplifier.
There have been many linearization technologies and algorithms developed to improve the nonlinear characteristics issues of high power amplifiers. Among them, a predistortion method, an envelope feedback method, and a feedforward method are well-known.
Recently the predistortion method has been widely used in mobile communication base stations because its structure is simple, and its efficiency is superior as compared to the feedforward method. Additionally, unlike the envelope feedback method, the predistortion method has no limit on bandwidth.
The basic principle of the predistortion method is to improve the linearity of a high power amplifier by distorting an input signal in advance contrary to nonlinear distortion character of the high power amplifier, and providing the predistorted signal to the power amplifier as an input. Thus, the amplifiers distortion neutralizes the predistortion, leaving an undistorted amplified signal.
FIG. 1 illustrates a related art predistortion linearizer for a high power amplifier.
As shown in FIG. 1, the related art predistortion linearizer comprises a first directional coupler 1, which samples some of an input signal, a phase shifter 2, which changes a phase of the input signal, and a variable attenuator 3, which changes a gain of the input signal. The related art device also includes a power amplifier 4, which amplifies an output of the variable attenuator 3, and a second directional coupler 5, which samples the output of the power amplifier. Finally, a comparison unit 6 is provided to control the phase shifter 2 and the variable attenuator 3 by comparing the output of the first and second directional couplers 1, 5.
An operation of the related art predistortion linearizer for the power amplifier will now be described.
The power amplifier 4 has nonlinear characteristics. In other words, its gain decreases and its phase is delayed in accordance with an increase of an input signal. Accordingly the nonlinear character of the power amplifier 4 can be improved by changing an input signal, so as to be an inverse of the gain decrease and phase change of the power amplifier 4. This is done using the phase shifter 2 and the variable attenuator 3.
When the phase shifter 2 and the variable attenuator 3 operate normally and the sampling rate is adjusted, signals outputted from the first and second directional coupler 1, 5 are equivalent. Herein, the sampling rate is determined so as to make both signals equivalent, and the sampling is performed in accordance with the output signal of the power amplifier 4.
The related art predistortion linearizer has various problems. For example, most of the related art phase shifters and variable attenuators are fabricated using FETs or diodes, and it is very difficult to fashion the nonlinear characteristics of the FET or diode to be accurately inverse to nonlinear characteristics of the power amplifier.
Accordingly, a difference of the signals outputted from the first and second directional coupler 1, 5, namely, the error value, is measured by the comparison unit 6, which includes an OP amp. Thus the phase shifter 2 and the variable attenuator 3 are controlled in accordance with the measured error value, and the nonlinear characteristics of the power amplifier 4 are compensated.
Using this method, however, the related art predistortion linearizer of the power amplifier cannot properly compensate when the nonlinear characteristics of the power amplifier change in accordance with time or external circumstances. Accordingly, the linearity of the power amplifier is reduced and remains uncompensated.
Specifically, as recited above, the phase shifter and the variable attenuator of the related art predistortion linearizer are diodes, and the comparison unit comprises OP-amps. As such, these analog circuits are greatly influenced by external circumstances (such as temperature and noise), and their accuracy is thus lower than a digital circuit.
In addition, an analog circuit can not accurately transmit a control voltage, and can not follow a response speed when the bandwidth of an input signal is wide. Accordingly, the linearity of the power amplifier lowers.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
An object of the present invention is to provide a predistortion linearizer for a power amplifier, which substantially obviates problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a predistortion linearizer for a power amplifier, which is capable of improving nonlinear character of the power amplifier quickly and accurately.
Another object of the present invention is to provide a predistortion linearizer for a power amplifier, which can maintain the linearity of the power amplifier when the nonlinear characteristics of the power amplifier change in accordance with external circumstances.
To achieve at least the above objects, in whole or in part, there is provided a predistortion linearizer for a power amplifier having a first and a second envelope detector which separately detect an envelope of input signals and output signals, a DSP which adjusts tally of a work function by comparing output signals of the first and the second envelope detectors, a work function generator, which generates a work function from the envelope of input signals and outputs gain and phase control voltages in accordance with the work function tally inputted from the DSP, and a vector modulator, which distorts input signals in accordance with the control voltage inputted from the work function generator and outputs the distortion compensation signal to the power amplifier.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.